High temperature characteristics of AlGaN/GaN high electron mobility transistors

Direct current (DC) and pulsed measurements are performed to determine the degradation mechanisms of AlGaN/GaN high electron mobility transistors (HEMTs) under high temperature. The degradation of the DC characteristics is mainly attributed to the reduction in the density and the mobility of the two-dimensional electron gas (2DEG). The pulsed measurements indicate that the trap assisted tunneling is the dominant gate leakage mechanism in the temperature range of interest. The traps in the barrier layer become active as the temperature increases, which is conducive to the electron tunneling between the gate and the channel. The enhancement of the tunneling results in the weakening of the current collapse effects, as the electrons trapped by the barrier traps can escape more easily at the higher temperature.     

Selective trap filling induced by electron pulse excitation during TSC measurement in PbI2

Thermally stimulated current (TSC) measurements have been performed on samples cut from a PbI₂ single crystal. After light excitation, several peaks have been observed in the range 77-320 K. It is demonstrated that excitation by means of a pulsed electron beam can be used to selectively fill either electron or hole traps. The results on PbI₂ confirm that most of the observed traps can be identified as hole traps.     

Direct-current and radio-frequency characteristics of passivated AlGaN/GaN/Si high electron mobility transistors

AlGaN/GaN/Si HEMTs grown by molecular beam epitaxy have been investigated using spectroscopy capacitance, direct and pulse current–voltage and small-signal microwave measurements. Passivation of the HEMT devices by SiO₂/SiN with NH₃ and N₂O pretreatments is made in order to reduce the trapping effects. As has been found from DLTS data, some of electron traps are eliminated after passivation. This has led to an improvement in the drain current. To describe the electron transport, we have developed a charge-control model by including the deep traps observed from DLTS experiments. The thermal and trapping effects have been, on the other hand, studied from a comparison between direct-current and pulsed conditions. As a result, a gate-lag and a drain-lag were revealed indicating the presence of deep lying centers in the gate-drain spacing. Finally, small-signal microwave results have shown that the radio-frequency parameters of the AlGaN/GaN/Si transistors are improved by SiO₂/SiN passivation and more increasingly with N₂O pretreatment.     

First photoconductivity measurement following photoionization of rare-earth dopant in a dielectric powder

Transient response of the dielectric permittivity of dielectric powder of LSO:Ce³⁺ under pulsed laser excitation were recorded by the 8-mm microwave resonator technique at room temperature. The signal in absorption mode is the signature of Ce³⁺ photoionization and photoconductivity effect in the rare-earth-doped dielectric grains. The signal in dispersion mode results from surface defects around each grain leading to electronic traps. This opens new perspectives for photoconductivity measurements on rare-earth-doped insulating nanopowders.     

Motions of the flux lines in high-T c superconductors from NMR linewidth,relaxation and spin echo decay

The motion of the flux lines (FL) in high temperature superconductors and their relationship with the NMR quantities are reviewed and discussed in the light of recent⁸⁹Y NMR experiments in YBCO-type compounds. In particular measurements involving the⁸⁹Y spin echo attenuation induced both by the thermal excitation of the FL’s and by motions driven by DC current and pulsed magnetic fields are presented, with preliminary results and lines of interpretation. Flux line motion as observed with¹⁹⁹Hg NMR in HgBa₂CuO_4+δ high temperature superconductor is discussed.     

Impact of proton irradiation with different fluences on the characteristics of InP/InGaAs heterostructure

ABSTRACT

The effect of traps to C–V and I–V plots of InP/InGaAs heterostructure with 3?MeV proton irradiation at different fluences has been discussed. After proton irradiation, the total reverse capacitance increases, which does not only include the variation of the depletion region width, but also the charging and discharging effect of traps. The total actual traps density N_SS of InP/InGaAs heterostructure could reach 13 orders of trap density, which is from the peak under reverse bias. The forward current is dominated by recombination current at low voltage and by the tunneling current at high voltage. The tunneling current and trap-assisted tunneling current are dominant in the reverse current.     

脉冲直流偏压增强的高质量立方氮化硼薄膜的合成

采用磁增强活性反应离子镀系统成功地合成了立方氮化硼薄膜.通过给基片施加脉冲直流偏压以代替传统的射频偏压,增强了立方氮化硼的成膜稳定性,研究了基片的直流脉冲偏压、等离子体放电电流、通入气体流量比(Ar/N₂)和基片温度沉积参数对立方氮化硼薄膜形成的影响规律.结果表明:随着基片负偏压和放电电流的增大,薄膜中立方氮化硼的纯度提高,当基片负偏压为155V,放电电流为15A时,可获得几乎单相的立方氮化硼薄膜.基片温度为500℃和Ar/N₂流量比为10时,最有利于立方氮化硼 关键词： 立方氮化硼 活性反应离子镀 脉冲偏压     

Generation of a pulsed high-current low-energy beam in a plasma electron source with a self-heated cathode

The transition of a low-current discharge with a self-heated hollow cathode to a high-current discharge is studied, and stability conditions for the latter in the pulsed–periodic mode with a current of 0.1–1.0 kA, pulse width of 0.1–1.0 ms, and a pulse repetition rate of 0.1–1.0 kHz are determined. The thermal conditions of the hollow cathode are analyzed, and the conclusion is drawn that the emission current high density is due to pulsed self-heating of the cathode’s surface layer. Conditions for stable emission from a plasma cathode with a grid acting as a plasma boundary using such a discharge are found at low accelerating voltage (100–200 eV) and a gas pressure of 0.1–0.4 Pa. The density of the ion current from a plasma generated by a pulsed beam with a current of 100 A is found to reach 0.1 A/cm². Probe diagnostics data for the emitting and beam plasmas in the electron source are presented, and a mechanism behind the instability of electron emission from the plasma is suggested on their basis.     

A study on Al2O3 passivation in GaN MOS-HEMT by pulsed stress

This paper studies systematically the drain current collapse in AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors （MOS-HEMTs） by applying pulsed stress to the device. Low-temperature layer of Al2O3 ultrathin film used as both gate dielectric and surface passivation layer was deposited by atomic layer deposition （ALD）. For HEMT, gate turn-on pulses induced large current collapse. However, for MOS-HEMT, no significant current collapse was found in the gate turn-on pulsing mode with different pulse widths, indicating the good passivation effect of ALD Al2O3. A small increase in Id in the drain pulsing mode is due to the relieving of self-heating effect. The comparison of synchronously dynamic pulsed Id - Vds characteristics of HEMT and MOS-HEMT further demonstrated the good passivation effect of ALD Al2O3.     

Alternating current electrical conductivity of high-density polyethylene-carbon nanofiber composites

High-density polyethylene (HDPE)-carbon nanofiber (CNF) composites with good dispersion of fillers in the polymer matrix were melt-compounded in a Haake mixer. The dependences of the alternating current conductivity of such nanocomposites on the filler content, temperature, and DC bias were investigated. The results showed that the electrical conducting behavior of HDPE-CNF nanocomposites can be well characterized by the direct current conductivity ( s_DC \sigma_{{{\rm DC}}}^{} , characteristic frequency (f_c) and critical exponent (s . It was found that s_DC \sigma_{{{\rm DC}}}^{} of percolating HDPE-CNF nanocomposites increases with increasing filler concentration and follows the scaling law of percolation theory. Increasing temperature caused a reduction of s_DC \sigma_{{{\rm DC}}}^{} , leading to the occurrence of positive-temperature-coefficient effect near the melting temperature of HDPE matrix. Application of DC bias led to an increase of s_DC \sigma_{{{\rm DC}}}^{} due to the creation of additional conducting paths within the polymer composites. The characteristic frequency generally followed the same tendency as s_DC \sigma_{{{\rm DC}}}^{} . The s values of percolating composites were slightly higher than those predicted by the percolation theory, indicating the presence of tunneling or hopping conduction in these composites.     

Kink effect in AlGaN/GaN high electron mobility transistors by electrical stress

The kink effect is studied in an AlGaN/GaN high electron mobility transistor by measuring DC performance during fresh, short-term stress and recovery cycle with negligible degradation. Vdg plays an assistant role in detrapping electrons and short-term stress results in no creation of new category traps but an increase in number of active traps. A possible mechanism is proposed that electrical stress supplies traps with the electric field for activation and when device is under test field-assisted hot-electrons result in electrons detrapping from traps, thus deteriorating the kink effect. In addition, experiments show that the impact ionization is at a relatively low level, which is not the dominant mechanism compared with trapping effect. We analyse the complicated link between the kink effect and stress bias through groups of electrical stress states: Vds = 0-state, off-state, on-state (on-state with low voltage, high-power state, high field state). Finlly, a conclusion is drawn that electric field brings about more severe kink effect than hot electrons. With the assistance of electric field, hot electrons tend to be possible to modulate the charges in deep-level trap.     

Nitriding of Stainless Steel in Electron-Beam Plasma in the Pulsed and DC Generation Modes

The influence of electron-beam parameters on the thickness and phase composition of a hardened layer formed upon the nitriding of austenitic stainless steel 12Cr18Ni10Ti in plasma produced by a beam in a low-pressure (3 Pa) nitrogen-argon mixture is studied. The results obtained in the DC and pulse-periodic modes of beam generation with the same mean current and electron energy are compared. In this case the negative bias voltage applied to the samples is 100 V. The nitriding temperature of 400°C is maintained at a mean beam current of 2.6 A and various combinations of frequency (100–500 Hz) and current pulse durations (0.1–0.3 ms) with an amplitude of 80 A. The mean ion-plasma current densities in the DC and pulsed modes are close in magnitude (2–3 mA/cm² at 400°C). The high pulsed ion-current density (35–70 mA/cm²) creates conditions under which the surface sputtering rate during the pulse exceeds the growth rate of the nitrided layer. The nitriding of steel in the pulsed and DC modes over four hours gives the same result. Hardened layers with a thickness of 7–8 μm and a microhardness of the surface component of 15 ± 1 GPa in which the main phase is a supersaturated nitrogen solid solution (expanded austenite) are formed. A possible explanation is that nitriding in an electron-beam plasma proceeds mainly under the action of long-lived active neutral nitrogen particles rather than as a result of ion bombardment.     

A new physics-based self-heating effect model for 4H-SiC MESFETs

A new self-heating effect model for 4H-SiC MESFETs is proposed based on a combination of an analytical and a computer aided design (CAD) oriented drain current model. The circuit oriented expressions of 4H-SiC low-field electron mobility and in-complete ionization rate, which are related to temperature, are presented in this model, which are used to estimate the self-heating effect of 4H-SiC MESFETs. The verification of the present model is made, and the good agreement between simulated results and measured data of DC I-V curves with the self-heating effect is obtained.     

Influence of the carbon substitution on the critical current density and AC losses in MgB<Subscript>2</Subscript> single crystals

The DC magnetization and AC complex magnetic susceptibilities were measured for MgB₂ single crystals, unsubstituted and carbon substituted with the composition of Mg(B_0.94C_0.05)₂. The measurements were performed in AC and DC magnetic fields oriented parallel to the c-axis of the crystals. From the DC magnetization loops and the AC susceptibility measurements, critical current densities (J _c were derived as a function of temperature and the DC and AC magnetic fields. Results show that the substitution with carbon decreases J _c ) at low magnetic fields, opposite to the well known effect of an increase of J _c at higher fields. AC magnetic losses were derived from the AC susceptibility data as a function of amplitude and the DC bias magnetic field. The AC losses were determined for temperatures of 0.6 and 0.7 of the transition temperature T _c , so close to the boiling points of LH₂ and LNe, potential cooling media for magnesium diboride based composites. The results are analyzed and discussed in the context of the critical state model.     

Laser heating in diamond anvil cells: developments in pulsed and continuous techniques

Developments in continuous and pulsed laser‐heating techniques, and finite‐element calculations for diamond anvil cell experiments are reported. The methods involve the use of time‐resolved (5 ns gated) incandescent light temperature measurements to determine the time dependence of heat fluxes, while near‐IR incandescent light temperature measurements allow temperature measurements to as low as 500 K. Further optimization of timing in pulsed laser heating together with sample engineering will provide additional improvements in data collection in very high P–T experiments.     

Fabrication and characterization of a PbTe quantum dots multilayer structure

Multilayer PbTe quantum dots (QDs) and SiO₂ were grown by pulsed laser deposition (PLD) and Plasma enhanced chemical vapor deposition (PECVD) techniques. The crystalline structure, QD size and size dispersion were observed by high-resolution transmission electron microscopy (HRTEM) measurements. This technique allows one to grow PbTe QDs as small as 1.8 nm diameter and 0.6 nm size dispersion. The whole structure can be used in a Fabry–Perot cavity for an optical device operating at the mid-infrared region.     

Hot carrier injection degradation under dynamic stress

In this paper,we have studied hot carrier injection(HCI) under alternant stress.Under different stress modes,different degradations are obtained from the experiment results.The different alternate stresses can reduce or enhance the HC effect,which mainly depends on the latter condition of the stress cycle.In the stress mode A(DC stress with electron injection),the degradation keeps increasing.In the stress modes B(DC stress and then stress with the smallest gate injection) and C(DC stress and then stress with hole injection under V g = 0 V and V d = 1.8 V),recovery appears in the second stress period.And in the stress mode D(DC stress and then stress with hole injection under V g = 1.8 V and V d = 1.8 V),as the traps filled in by holes can be smaller or greater than the generated interface states,the continued degradation or recovery in different stress periods can be obtained.     

大功率GaN基发光二极管等效串联电阻的功率耗散及其对发光效率的影响

由于自加热效应的存在,大功率GaN基发光二极管(LED)的芯片温度有可能高出环境温度很多,实验中,芯片温度超出环境高达147 K.从实验测量的大功率LED电流电压特性曲线中,将p-n结和等效串联电阻上的电压降落分离出来,得到了大功率LED等效串联电阻随芯片温度的变化情况.在输入电功率自加热效应的影响下,大功率GaN基LED等效串联电阻呈现出剧烈的变化,其阻值由低输入功率时的1.2 Ω降低到0.9 Ω,然后再升高到1.9 Ω,等效串联电阻的功率耗散在输入功率中所占的比例也随着输入功率的增加迅速增加,最高时接 关键词： 自加热 等效串联电阻 发光二极管 流明效率     

GaS0.4Se0.6: Relevant properties and potential for 1064 nm pumped mid-IR OPOs and OPGs operating above 5 μm

We present measurements of the transparency, refractive index dispersion, nonlinear coefficient, damage threshold, and two-photon absorption of mixed GaS _x Se_{1 − x} crystals and show that GaS_0.4Se_0.6 is a promising nonlinear material for down conversion of pulsed 1064 nm radiation to the mid-IR above 5 μm without significant two-photon absorption.     

Characterization of N-doped ZnO layers grown on (0 0 0 1) GaN/Al2O3 substrates by molecular beam epitaxy

We investigated the optical properties and electrical properties of N-doped ZnO layers grown on (0 0 0 1) GaN/Al₂O₃ substrates by molecular beam epitaxy, employing 10 K photoluminescence (PL) measurements, current–voltage (IV) measurements, capacitance–voltage (CV) measurements, and 100 K photocapacitance (PHCAP) measurements. 10 K PL spectra showed that excitonic emission is dominant in N-doped ZnO layers grown after O-plasma exposure, while overall PL emission intensity is significantly reduced and deep level emission at around 2.0 2.2 eV is dominant in N-doped ZnO layers grown after Zn exposure. IV and CV measurements showed that N-doped ZnO layers grown after Zn exposure have better Schottky diode characteristics than O-plasma exposed samples, and an N-doped ZnO layer grown at 300 °C after Zn exposure has best Schottky diode characteristics. This phenomenon is presumably due to lowered background electron concentration induced by the incorporation of N. PHCAP measurements for the N-doped ZnO layer revealed several midgap trap centers at 1.2 1.8 eV below conduction band minimum.